Hi Albrecht - see within.
I've copied this onto the reflector.
----- Original Message -----
Sent: Tuesday, April 26, 2005 2:57
AM
Subject: TP3 noise discussion
Hello Tom,
For
today's TP3 call, Jim asked us to look at the noise
calculation
again. Therfore, I would like to share my
thoughts on this topic with
you. May be I am missing
something completely, which is the reason
why I am
sharing my thoughts with you and not sending
them
directly to the LRM reflector.
Our main concern about the
TP3 SNR definition is
that
we don't see the relationship between the link
budget
and the SNR calibration as
currently defined by the Qsq ratio
of
11.5 in the LRM draft.
Regarding the link budget, the LRM community is
assuming a
reference SNR at the
input to the EDC of 30dBe (17+2x6.5).
This
value is used for TWDP calculations and for
PIE-D
coverage work and assumes a
reference channel without ISI
but with RIN and MN
penalties.
TL - this particular calculation (for TWDP) does
not include RIN or MN. It is based on a matched filter bound
receiver that provides -13 dBm optical sensitivity to a perfect
rectangular waveform. The min received OMA is -6.5 dBm, so the
difference is 6.5 dB (and hence, the value in the
equation).
Having written this, I think the value may
have a problem. Perhaps the SNR without these terms should be 32 dBe.
The latest budgets (see Nov 04) show sensitivities at ~-14 dBm (not
-13), required to accommodate 1 dB of penalties due to RIN and
MN!
For
the TP3 SNR calibration, the value of Qsq
is
derived from 0.9dBo noise power
penalty. Even Piers
pointed out in
dawe_1_0504 that the contribution from
MN
of 0.5dB is pessimistic as it was
defined
for GbE where the exact value did
not matter and OFL was
used. We further noticed that a slight change
from 0.9 to 1.0dBo
will have a large
impact on Qsq (11.5 to 12) and the resulting SNR.
TL - the penalties were set at 0.5 and 0.4. They are
independent and assume no other simultaneous
noises. These cannot be simply added when computing penalties.
Mathematically, they combine to 1.0 dB. The math is right, but if
the values for the individual terms are too high, then we should lower
them (which will reduce the
combination).
The
SNR corresponding to Qsq=11.5 is 20log11.5 =
21.2dBe
Using Lew's assumption of -3dBo
noise coloring due to IFR,
this improves the SNR
at the optical receiver input to
about
27.2dBe. Assuming a reciever sensitivity of -14dBm and an OMA
of
-6.5dBm, this results in a SNR at the EDC
input of about 25.8dBe,
which is
significantly below the 30dBe
reference SNR derived
from the link
budget. To make the link budget and the
Qsq approaches
match, you would
need to reduce MN+RIN or increase IFR.
Should these match at
all?
TL - I do think there is a problem. For example, the RIN
spec produces Qsq = ~29 at TP2. MN may bring it down further, maybe
~20 - not 11.5. 0.5 and 0.4 came from experience at 802.3ae, where
an overall Qsq on the order of 20 produces penalties in the overall
budget of 1 dB due to interactions with other impairments, mostly ISI
closure. We should not be starting with 1 dB, but ending there. Now,
there is the question of whether any of these are the right values for
802.3aq! Perhaps the 29 factor is fine for RIN at TP2, and perhaps 0.4
dB for MN is okay (at TP3, not a TP2), but the interaction with all the
dispersion is not well understood. That's the rub here. Paul Voois
did some early spreadsheet work on RIN, and that work should be
revisited. See http://grouper.ieee.org/groups/802/3/aq/public/may04/voois_1_0504.pdf.
I'll try to take some time to look at
this work. Finally, I wonder why we can ignore jitter
during the stress test,when
we needed to apply it for 10G-Base-LR .
TL - if we reduce the
noise, we may need to add some jitter back in, although the added
complexity is not desirable. Currently, the noise is high, so no added
jitter is needed.
--
Best regards,
Albrecht Rommel
Acuid Corporation Ltd.
Phone +49 (0)8141 3555 813
Fax +49 (0)8141 3555 829
Mobile +49 (0)172 607 8039
albrecht.rommel@acuid.de
www.acuid.com